1. Field of the Invention
The present invention relates to a disk recording system for recording data onto a disk using a light beam emitted from an optical head, and more particularly to a disk recording system which ensures continuity of recorded data.
2. Description of Related Art
CD-R (Recordable) drives or CD-RW (ReWritable) drives of the CD (Compact Disk) family are well known disk recording systems which record digital data on disks using laser light emitted from an optical head.
In such disk recording systems, recording speed or data stream conditions may lead to buffer underrun errors, which in turn causes the recording operation to be halted when generation of data to be recorded on the disk is interrupted.
For write-once media such as CD-R disks, a disk can no longer be used once the recording operation is halted.
For rewritable disks such as CD-RW disks, the disk remains usable, but, when recording is interrupted and resumed, recording must be repeated from the beginning.
A packet write system which records data in packet units provides one solution to this problem. The packet write system, however, requires as many as seven link blocks for connecting the packets and the disk recording capacity used in this manner is wasted.
Accordingly, when recording operation is interrupted and then resumed, if recording data can be recorded so as to be continuous to data already recorded on the disk prior to the interruption of recording without providing any link blocks in a case of track at once or session at once system, such a system would be preferable in light of compatibility with CD-ROM drives. Further, compared with a recording system which requires link blocks for connecting data when recording is resumed such as a packet write system, continuous data recording which requires no link blocks is more advantageous in terms of effective use of disk recording capacity. For these reasons, a disk recording system which can write recording data without requiring the use of link blocks has been in demand.
For continues data recording as described above, it is not only necessary that additional recording data be written so as to be continuous to data already recorded on a disk, but also that the start of data writing be synchronized with the data already recorded. As a result, the data linking processes are complicated.
In order to synchronize additional data to be written with data already recorded, it is necessary to extract a bit clock from data read from pits on the disk which form the recorded data, and to encode the recording data in accordance with the bit clock. Once writing starts, on the other hand, it is necessary to generate a more accurate reference clock synchronized with the bit clock so that the start of data recording is begun using the reference clock.
It is thus necessary to switch a system clock which functions as an operation clock used for a general operation regarding disk recording and reproducing between a bit clock and a more accurate reference clock. Specifically, during the operation for synchronizing data to be written with the recorded data, the system clock is a bit clock, which is switched to a reference clock when writing is started.
However, when the system clock is directly switched from a bit clock to a reference clock by a selection circuit when data writing is started, the following problem occurs. Namely, due to a phase deviation and speed variation, recording data which includes disturbance in clock components is written on a data portion before and after a link portion of the recording data. This will lead to an increased reproduction error at the data portion before and after the data link portion at the time of disk reproducing.
Further, direct switching from a bit clock to a reference clock by a selection circuit also creates a problem that peak signal disturbance is generated at the point of switching.
According to the present invention, a system clock generated from a system clock generation circuit is gradually switched from a state which is synchronized with a bit clock reproduced from pits of the recorded data to a state which is synchronized with a reference clock, and recording data is written on a disk in accordance with the system clock. It is thus possible to prevent data including significant disturbance in clock components from being recorded before and after data link portions of the disk to thereby reduce a reproduction error when the data portion before and after the data link portion is reproduced.
In particular, according to one preferred aspect, a system clock generation circuit comprises a single system PLL circuit, such that gradual switching of a system clock from a state which is synchronized with a bit clock to a state which is synchronized with a reference clock can be achieved with a simple structure.
In accordance with another aspect of the present invention, when a PLL circuit of a system clock generation circuit is locked after resumption of writing, a system clock generated from the system clock generation circuit is switched from a reference clock generated through the PLL circuit to a reference clock generated not through the PLL circuit. It is therefore possible to prevent adverse effect on the recording operation due to jitter components generated through the PLL circuit.